This invention relates to a method of doping impurities uniformly into a sidewall and bottom of a trench formed in a semiconductor substrate.
By forming a trench in a semiconductor substrate, and doping impurities in its sidewall to form a part of a semiconductor device, the two-dimensional area can be utilized three-dimensionally, and a submicron device may be formed. So far, various methods have been proposed as a means of doping impurities into sidewall and bottom of a trench: for example, (i) the method of doping by ion implantation from above by obliquely forming a sidewall of trench (T. Shibata et al., IEDM Tech. Dig., pp. 27-30, Dec. 1983), (ii) the method of doping by using thermal diffusion by AsSG (K. Yamada et al., IEDM Tech. Dig., pp. 702-705, Dec. 1985), and (iii) the method of doping uniformly into a sidewall by ion implantation (G. Fuse et al., VLSI Symposium Dig. of Tech. Papers, pp. 58-59). However, these methods have their own disadvantages. First, in the case of forming a trench having a tapered sidewall and doping impurities by ion implantation, it is impossible to reduce width from a certain limit because of following limitations on depth and width. That is, the minimum trench width W.sub.I may be limited by 2D tan .alpha., where D and .alpha. are trench depth and taper angle, respectively, so that, for example, W.sub.I is limited to be about 0.69 .mu.m for D=0.6 .mu.m and .alpha.=60.degree.. Therefore, it is impossible to use as a trench sidewall having a high aspect ratio. Next, concerning the method of using AsSG, since a thermal process is required, there may occur a problem as to mask material in doping selectively, and it may be impossible to control in low concentration impurity doping. Finally, in the doping method depending on tilt angle of implantation, since the shadow area cannot be doped with impurities by ion implantation, it is necessary to emit ion beams by rotating the substrate. Or, when implanting into the whole circumference of the sidewall, four-fold implantation is necessary by rotating the substrate by 90 degrees each. Furthermore, in ion implantation, since the directivity is very strong, the concentration in a bottom of a trench is extremely higher than that in a sidewall in the ion implantation into a trench having the vertical sidewall. It is hence difficult to dope uniformly into a trench for forming a trench capacitor of dynamic random access memory (D-RAM) or the like.
Recently plasma doping has been reported by Ishiwata et al. [O. Ishiwata, Y. Itoh, N. Sato, Y. Seki, K. Matsuzaki, and S. Ishida, Extended Abstracts (Spring Meeting, 1987; The Japan Society of Applied Physics), pp. 516] They doped impurities into Si at a relatively lower temperature of 200.degree. C. and under the pressure of 2 torr for a DC glow discharge. However, in this report, too, impurity doping into the trench inner wall is not suggested.